1. Field of the Invention
The present invention relates to a photovideo camera and, more specifically, to a photovideo camera capable of determining a desired angle of view by zooming before focusing, and capable of focusing without changing the desired angle of view.
2. Description of the Prior Art
Recently, so-called photovideo cameras have widely been used for displaying on a monitor or for recording on a video tape a visible image formed on a negative film or a positive film by converting the visible image into video signals.
The photovideo camera comprises a film stage for supporting a film, such as a negative film or a positive film, a light source disposed under the film stage to illuminate the film supported on the film stage, a lens unit for focusing the light projected by the light source through the visible image, a solid-state image pickup device, such as a CCD (charge coupled device), for converting the image formed by the lens unit into corresponding video signals, a video signal inverting circuit for inverting the video signal to provide positive video signals when the visible image is negative, and a turning mechanism for turning the solid-state image pickup device in a fixed range in a plane parallel to its focal plane.
The lens unit has a zoom lens system for varying magnification, and a focusing lens system for focusing. Generally, the front lens, namely, a group of lenses on the side of the object, of the focusing lens system is moved along the optical axis for focusing. Such a focusing system is called an outer focusing system. FIGS. 5 to 7 show a lens unit a of an outer focusing system. As shown in FIGS. 5 to 7, a front lens b is held for movement along the optical axis in the lower end of a lens barrel, not shown. A zoom lens system c consists of a variator d disposed above the front lens b, and a compensator e disposed above the variator d. The variator d and the compensator e are held individually for movement along the optical axis in the lens barrel. A relay lens system f is held fixedly above the compensator e in the lens barrel to focus an image transmitted through the compensator e on the focal surface of a solid-state image pickup device g disposed above the relay lens system f. The front lens b, the variator d, the compensator e and the relay lens system f are arranged with their optical axes in alignment with the optical axis of the photovideo camera. The solid-state image pickup device g is disposed with its focal surface extended perpendicularly to the optical axis and with its center on the optical axis of the photovideo camera.
The front lens b is moved along the optical axis for focusing. For example, when the lens unit a is in a front focus state, namely, when the lens unit a is focused on an object nearer to the lens unit than a desired object, the front lens b is moved upward for focusing, and when the lens unit a is in a rear focus state, namely, when the lens unit a is focused on an object further from the lens system than the desired object, the front lens b is moved downward.
The variator d and the compensator e are moved individually along the optical axis for zooming. The movement of the variator d changes magnification, and the movement of the compensator corrects the deviation of the focus point. Magnification is dependent on the distance between the variator d and the front lens b.
Suppose that the lens system a is in a standard state where the front lens b, the variator d and the compensator e are disposed as shown in FIG. 5. Then, the lens unit a is brought to a wide-angle state when the variator d is moved downward (toward the front lens b) from the position in the standard state shown in FIG. 5 as shown in FIG. 6. When the variator d is moved toward the front lens b, the compensator e is moved slightly upward (away from the front lens b) to correct the deviation of the focus point resulting from the downward movement of the variator d. When the variator d is moved upward (away from the front lens b) from the position in the standard state, the lens unit a is brought to a telephoto state as shown in FIG. 7, in which magnification is increased. When the variator d is moved upward, the compensator e is moved slightly upward to correct the deviation of the focus point resulting from the upward movement of the variator d.
The variator d and the compensator e are interlocked with each other so that the compensator e is moved to correct the deviation of the focus point when the variator d is moved. Accordingly, once the lens unit a is focused, theoretically, it is not necessary to focus the lens unit a again after zooming.
However, in actual practice some correction of the focus point is necessary after zooming. Since the out of focus condition is more conspicuous in a telephoto state than in a wide-angle state, the lens unit a must be focused again if the lens unit a is zoomed up after focusing. The recent trend toward greater zoom ratio has further enhanced the importance of focus correction after zooming.
Since the films to be mounted on the film holder of the photovideo camera are different from each other in curvature on the film holder, the distance between the film mounted on the film holder and the front lens b changes when the film is changed. Therefore, in most cases, the focus of the lens unit a must be adjusted every time the film is changed.
The conventional photovideo camera of an outer focusing system has problems specific thereto. If the front lens b is moved along the optical axis for focusing after enlarging a visible image on a film in a desired angle of view for trimming, the desired angle of view changes, which is due to the change of the magnification resulting from the change of the distance between the front lens b and the variator d. Therefore, if the front lens b is moved for focusing after trimming the visible image on a film h so that an area i surrounded by thick solid lines in FIG. 8 is included in a desired angle of view, the desired angle of view is changed to include an area j surrounded by alternate long and two short dashes lines in FIG. 8.